1. Field of the Invention
The present invention relates in general to the production of amorphous, low-carbon content (hence, off-white in color) siliceous ash from, e.g., rice hulls and other siliceous waste materials. More particularly, the present invention relates to a method of producing a low-carbon content, amorphous siliceous ash that is suitable to form zeolites structures and high-performance cement, and a composition of siliceous ash thereof.
2. Description of the Prior Art
Siliceous waste materials from agricultural products, especially rice hulls, can be used for a variety of applications. However, processing of the siliceous waste material is typically necessary to enhance its physical and chemical properties. The silicon, typically in the form of silicon dioxide (SiO2), derives from the cell structure of plant material. Rice hulls, for example, are generally found to have about 20% SiO2 by weight. Given the industrial sources and nature of carbon thermal treatment, there are differences in carbon content and forms of carbon (i.e., graphitic or amorphous), and thus a need for purification of the SiO2 from the raw siliceous waste materials such as rice hulls that retains the mineralogy of the amorphous SiO2.
The disposition of siliceous waste materials such as rice hulls has become a substantial problem in the agricultural industry in that rice hulls typically can only be disposed of by burning or burying. This presents environmental problems, as landfilling is limited, and burning the waste material in an uncontrolled manner is both wasteful and presents undesirable pollutants. Additionally, the presence of crystalline SiO2, e.g., quartz, cristobalite, may also pose a health hazard such as silicosis. Some potential uses for the SiO2 in plant matter could be used as pozzolan in cement, or use as a catalytic matrix such as zeolites.
When heating or pyrolyzing SiO2, there is a transition from an amorphous, non-crystalline form of material to a crystalline form know as tridymite and cristobalite. This takes place at a high temperature, such as 800xc2x0 C. to 1000xc2x0 C., for pure SiO2, and lower temperatures for impure SiO2, such as found in rice hulls and other silicon-containing plant waste. The crystallized form of SiO2 is not reactive, and hence, less useful as an end product than the amorphous form of SiO2. Retention of plant amorphous SiO2 is thus favorable. However, given the presence of carbon in the plant cell matrix in, for example, rice hulls, the conversion to purely low-carbon, off-white, amorphous SiO2 is difficult.
It has been discovered that a highly useful amorphous form of SiO2 may be produced by the careful, controlled incineration of rice hulls at relatively low temperatures and oxidizing atmospheres. Goodwin et al. (U.S. Pat. No. 4,571,389) disclose a method of making silicate aggregate, wherein rice hull ash with a carbon content of no more than 4% is reacted with boric acid, an alkali metal hydroxide in aqueous solution and heated. The result is a silicon-based aggregate useful as a sorbent. Sugita (U.S. Pat. No. 5,329,867) discloses a method of making an activated rice hull ash using a rotary-type kiln that allows for continuous air purging from the bottom of the reactor. Pitt (U.S. Pat. No. 3,959,007) discloses a method and apparatus for converting rice hulls to a highly amorphous form of SiO2 or rice hull ash. The apparatus disclosed by Pitt comprises a complex circular furnace wherein the introduced rice hulls are pyrolyzed to a temperature of up to 1300xc2x0 F., a flow of air causing a helical movement of the rice hull particles as they are pyrolyzed. A low-carbon content rice hull ash is obtained when the rice hulls are heated for 66 hours at 1100xc2x0 F. (ca. 600xc2x0 C). Given the complexity of this method and the high energy requirements (high temperature for prolonged periods of time), it is desirable to improve upon this method of forming a low-carbon content (hence, off-white) amorphous siliceous ash.
It is therefore an object of the present invention to provide a method of producing low-carbon content (0 to 3% by weight), and hence an off-white color, siliceous ash, the primary starting material for the ash being silicon-containing (siliceous) waste material.
It is another object of the present invention to provide a siliceous ash that can be used in cement, and as a filler in polymer compositions.
It is yet another object of the present invention to provide a siliceous ash that will improve the durability and strength of cement compositions.
It is yet another object of the present invention to provide a siliceous ash that is pure enough to be used in the synthesis of zeolites as catalysts.
It is yet another object of the present invention to provide a method of using siliceous waste material in a cost-added product, thus improving ecological problems caused by the landfilling and/or incineration of siliceous waste materials.
It is yet another object of the present invention to provide a method of producing siliceous ash from siliceous waste material, wherein the carbon content can be varied from 0% to 3% or more by weight of the ash. Further, it is an object to provide ash having amorphous carbon.
These objects are achieved by providing a method of producing low-carbon, amorphous siliceous ash from siliceous waste material, wherein a portion of waste materials is pyrolyzed in a furnace. The waste materials can be such things as rice hulls and other plant waste materials, including siliceous rice hull residue derived from bio-gas generation, and other sources of amorphous SiO2. One such furnace that can be used is a tube furnace, but other furnaces would also be suitable. For purposes of illustration, a tube furnace is described in FIG. 1, and the method of the invention is described with reference to a tube furnace. It is to be understood that any furnace having a known volume is suitable. The material is pyrolyzed in a tube furnace at a temperature of between about 500xc2x0 C. and 800xc2x0 C. for at least 0.5 to 6 hours or more. Simultaneous to the pyrolysis, a continuous stream of air is passed over the material, and gaseous products from the pyrolysis are drawn off. The siliceous ash is thus formed. The furnace can be provided with a means of continuously feeding siliceous waste material into the furnace, and conveying the siliceous ash out of the furnace.
The ratio of the weight of the portion of plant waste material to the volume of the tube furnace should be maintained at between about 0.02 and 0.10. Further, the siliceous ash product may have a carbon content of between about 0% and 3% by weight or more of the siliceous ash and a SiO2 content of at least 94% by weight of the siliceous ash.
Additional objects, features and advantages will be apparent in the written description which follows.